An isocentric system typically includes a radiation head mounted on a rotatable gantry. A radiation source is disposed in the radiation head and produces a radiation beam. The gantry rotates about a rotational axis such that the radiation beam intersects the rotational axis at an isocenter. A patient, supported on a couch or table (the terms being used interchangeably), is placed such that a target to be irradiated in the patient is at the isocenter, which enables irradiating the target from various angles. A typical distance between the radiation source and the isocenter of an isocentric linear accelerator (LINAC) system is 1 m.
Clearance is the distance between the radiation head and the patient or the couch, whichever is smaller. Such patient-dependent clearance may vary with gantry rotation and/or with couch positioning. Insufficient clearance for a range of gantry angles amounts to a collision between the radiation head and the patient or couch. The angular positions of the gantry where collision would occur are thus excluded from the arc treatment plan. Attaching external devices such as collimators or wedges to the radiation head further reduces the clearance and may exclude a wider range of gantry angles from being used in the arc treatment plan.